1. Field of the Invention
This invention relates to thin film metal alloy magnetic recording media, and in particular to a multilayered thin film metal alloy medium for vertical magnetic recording in which an alloy comprising cobalt and platinum forms the magnetic film in each of the layers.
2. Description of the Prior Art
Cobalt-based alloys which are known as capable for use in horizontal magnetic recording include cobalt-nickel (CoNi), cobalt-rhenium (CoRe), cobalt-palladium (CoPd) and cobalt-platinum (CoPt). In such media, the hexagonal-close-packed (HCP) crystalline structure of the cobalt alloy is formed on the substrate, or on an intermediate underlayer, so that the C-axis, i.e. the [002] axis, of the cobalt alloy film is either in the plane of the film or has a component in the plane of the film.
It has been found that in the case of CoPt films for horizontal recording, the horizontal coercivity (H.sub.c) is dependent upon the composition of the platinum, with the maximum H.sub.c occurring at approximately 20 atomic percent (at. %) platinum. See J. A. Aboaf, et al., "Magnetic Properties and Structure of Co-Pt Thin Films", IEEE Trans on Magnetics, MAG-19, 1514 (1983), and M. Kitada, et al., "Magnetic Properties of Sputtered Co-Pt Thin Films", J. Appl. Phys. 54 (12), December 1983, pp. 7089-7094. The horizontal coercivity and other properties of cobalt-platinum films have been reported by Opfer, et al., in an article entitled "Thin-Film Memory Disc Development," Hewlett-Packard Journal, November 1985, pp. 4-10.
In order to improve the coercivity of the CoPt magnetic film in certain types of disks for horizontal recording, a chromium (Cr) or chromium alloy underlayer may be formed between the substrate and the CoPt magnetic layer. The use of a Cr underlayer in a CoPt thin film disk is described in the above-referenced article by Opfer, et al., and in European patent application No. 145157, published June 19, 1985 and assigned to the Hewlett-Packard Company. The use of a chromium-cobalt (CrCo) alloy as an underlayer for various types of magnetic layers, including CoPt, is suggested in European patent application No. 140513, published May 8, 1985 and assigned to the same assignee as this application.
In contrast to thin film horizontal recording media, the thin film metal alloy for vertical magnetic recording is typically a single magnetic layer having an ordered crystalline structure with perpendicular magnetic anisotropy. The conventional material for vertical recording is a cobalt-chromium (CoCr) alloy which is sputter-deposited on the substrate, or on an intermediate nucleating layer deposited on the substrate, to form a film having an HCP crystalline structure with the C-axis oriented generally perpendicular to the substrate. The magnetic properties of thin film metal alloy vertical recording media made with various substrates, a titanium (Ti) nucleating layer and a single-layer CoCr perpendicular magnetic film are described by Kobayashi, et al. in "High Density Perpendicular Magnetic Recording on Rigid Disks", Fujitsu Scientific & Technical Journal, Vol. 19, No. 1 (March 1983), pp. 99-126. Great Britain patent application No. 2125069, published Dec. 29, 1983, describes a thin film disk for vertical recording which uses a single layer of cobalt-chromium-platinum (CoCrPt), with Pt comprising between 1 and 5 at. %.